Herpes stromal keratitis (HSK) in the mouse is regulated by CD4+ T lymphocytes when activated by Langerhans cells (LC) in the infected cornea. The PI proposed that a three-step interaction of CD+4 T cells with LC leads to HSK: 1) T cell receptor ligation by viral peptides that are bound to MHC class II molecules on LC; 2) binding of CD40 ligand (CD40L) on partially activated T cells to CD40 on LC induces LC expression of the costimulatory molecule B7-1 and the cytokine IL-12; 3) B7-1 binding to CD28 in the presence of IL-12 induces Th1 cytokine production by the CD4+ T cells. He proposes two specific aims to test this hypothesis. Specific aim 1 will expand on preliminary finding suggesting that B7-2 costimulation is requires for induction of the CD4+ T cell response to HSV in the lymph nodes, whereas B7-1 costimulation is required for re- stimulation of the CD4 + cells in the infected cornea. He will attempt to alter the inductive and effector phases of the CD4+ T cell response to HSV through intraperitoneal (i.p.) and subconjunctival (s.c.) Injection, respectively, of monoclonal antibodies (mAb) to B7-1, B7-2, and CTLA4. Cytokine production in the lymph nodes and corneas, and HSK will be monitored in these mice. In addition, the susceptibility to HSK of mice that are genetically deficient in B7-1, B7-2, and CD28 will be tested, and the relative importance of costimulation through B7-1 and B7-2 binding to CD28 in the lymph nodes and cornea will be determined through a series of adoptive transfer studies. Specific Aim 2 will determine the importance of T cells and the CD40L in regulating B7-1 expression and IL-12 production by LC in infected corneas, as well as the role of CD40L/CD40 interaction and IL-12 production in HSK. These vivo studies will involve blocking experiments with mAb to CD40L and IL-12, and adoptive transfer studies with mice that are genetically deficient in CD40L. These studies will define the costimulatory requirement for CD4+ T cell activation in the HSV-1-infected cornea, and possibly provide important new avenues for intervention in HSK.